Recently, various standards have been developed for data communication over broadband wireless links. One such standard is set out in the IEEE 802.16 specifications and is commonly known as WiMAX. The specifications include IEEE 802.16-2004, primarily intended for systems having fixed subscriber stations, and IEEE 802.16e-2005 which among other things provides for mobile subscriber stations. In the following description, the term mobile station (MS) is used as shorthand for both mobile and fixed subscriber stations. The term “user” is also used equivalently to mobile station.
The entire contents of IEEE Std 802.16-2004 “Air Interface for Fixed Broadband Wireless Access Systems” and IEEE Std 802.16e-2005 “Amendment 2 and Corrigendum 1 to IEE Std 802.16-2004” are hereby incorporated by reference. IEEE 802.16 defines wireless communication systems in which the mobile stations communicate with a base station within range, the range of a base station defining at least one “cell”. By deploying base stations at suitable positions within a given geographical area, and/or by providing multiple antennas in the same base station, a contiguous group of cells can be created to form a wide-area network. In this specification, the terms “network” and “system” will be used equivalently.
In systems of the above type, data is communicated by exchange of packets between the mobile stations and base station whilst a connection (management connection or transport connection) is maintained between them. Below, the term “channel” is sometimes used to refer to such a connection between one MS and its BS, as should be clear from the context. The direction of transmission of packets from the subscriber station to the base station is the uplink (UL), and the direction from the base station to the subscriber station is the downlink (DL). The packets have a defined format which follows a layered protocol applied to the system and its component radio devices. Protocol layers relevant to packets as such are the so-called physical layer (PHY) and media access layer (MAC). In the IEEE 802.16-2004 specification, these protocol layers form a protocol “stack” as shown in FIG. 1.
The media access layer shown in FIG. 1 is the protocol layer of most concern in the invention to be described. It is responsible for handling various functions including network access, bandwidth allocation, and maintaining connections. This includes controlling access of the BS and SS's to the network on the basis of “frames” which are the predetermined unit of time in the system, and which are divided in the time and frequency domain into a number of slots, and when utilising multiple transmit antennas may also be divided spatially into a number of streams.
Various physical layer implementations are possible in an IEEE 802.16 network, depending on the available frequency range and application; for example, a time division duplex (TDD) mode and a frequency division duplex (FDD) mode as described below. The PHY layer also defines the transmission technique such as OFDM (orthogonal frequency division multiplexing) or OFDMA (orthogonal frequency division multiple access), which techniques will now be outlined briefly.
In OFDM, a single data stream is modulated onto N parallel sub-carriers, each sub-carrier signal having its own frequency range. This allows the total bandwidth (i.e. the amount of data to be sent in a given time interval) to be divided over a plurality of sub-carriers thereby increasing the duration of each data symbol. Since each sub-carrier has a lower information rate, multi-carrier systems benefit from enhanced immunity to channel induced distortion compared with single carrier systems. This is made possible by ensuring that the transmission rate and hence bandwidth of each subcarrier is less than the coherence bandwidth of the channel. As a result, the channel distortion experienced on a signal subcarrier is frequency independent and can hence be corrected by a simple phase and amplitude correction factor. Thus the channel distortion correction entity within a multicarrier receiver can be of significantly lower complexity of its counterpart within a single carrier receiver when the system bandwidth is in excess of the coherence bandwidth of the channel.
An OFDM system uses a plurality of sub-carrier frequencies which are orthogonal in a mathematical sense so that the sub-carriers' spectra may overlap without interference due to the fact they are mutually independent. The orthogonality of OFDM systems removes the need for guard band frequencies and thereby increases the spectral efficiency of the system. OFDM has been proposed and adopted for many wireless systems. In an OFDM system, a block of N modulated parallel data source signals is mapped to N orthogonal parallel sub-carriers by using an Inverse Discrete or Fast Fourier Transform algorithm (IDFT/IFFT) to form a signal known as an “OFDM symbol” in the time domain at the transmitter. Thus, an “OFDM symbol” is the composite signal of all N sub-carrier signals. At the receiver, the received time-domain signal is transformed back to frequency domain by applying Discrete Fourier Transform (DFT) or Fast Fourier Transform (FFT) algorithm.
OFDMA (Orthogonal Frequency Division Multiple Access) is a multiple access variant of OFDM. It works by assigning a subset of the sub-carriers to an individual subscriber. This allows simultaneous transmission from several users leading to better spectral efficiency. However, there is still the issue of allowing bidirectional communication, that is, in the uplink and download directions, without interference.
In order to enable bidirectional communication between two nodes, two well known different approaches exist for duplexing the two (forward or downlink and reverse or uplink) communication links to overcome the physical limitation that a device cannot simultaneously transmit and receive on the same resource medium. The first, frequency division duplexing (FDD), involves operating the two links simultaneously but on different frequency bands by subdividing the transmission medium into two distinct bands, one for DL and the other for UL communications. The second, time division duplexing (TDD), involves operating the two links on the same frequency band, but subdividing the access to the medium in time so that only the DL or the UL will be utilizing the medium at any one point in time. Although both approaches have their merits and the IEEE802.16 standard incorporates both an FDD and TDD mode, the remainder of this description will mainly refer to the TDD mode.
FIGS. 2 and 3 illustrate the TDD frame structure used in the OFDM/OFDMA physical layer mode of the IEEE802.16 standard (WiMAX).
Referring first to FIG. 2, this shows an OFDM TDD frame structure from a packet perspective. The UL subframe shown in FIG. 2 first provides a contention slot for initial ranging. Initial ranging is the process by which an MS is admitted to the network, and involves the MS using the slot, on a contended basis (that is, in competition with any similar ranging requests from other users) to send an ID code to the BS, commonly referred to as a CDMA based ranging request. The BS responds to receipt of a ranging code by making a resource allocation on the next UL subframe. Although the BS does not yet know which MS made the CDMA based ranging request, the intended recipient is able to use the allocated resource to reply with a message (typically RNG-REQ, a MAC management message as referred to below) to identify itself and start off the network entry procedure.
The next resource allocated within the UL subframe is a contention slot for BW requests, which is used by any of the subscriber stations (again on a contended basis) to request bandwidth from the base station. A similar procedure may be followed as just outlined for initial ranging.
FIG. 2 further illustrates, within the UL-subframe, a packet format having two parts, a PHY header and a MAC PDU. The MAC PDU in turn consists of a MAC header, an optional payload, and optional error check code (cyclic redundancy code or CRC). The PHY header includes training sequences. Within the MAC PDU, the MAC header normally gives essential parameters for media access, such as the type of header, CID, and format of the PDU (e.g. whether it is encrypted, length) etc. The CRC within MAC PDU is optional, and can be used to check the received MAC PDU. The payload within MAC PDU is used to contain the data which the MS wishes to send to the BS, but is also optional. For example, some controlling messages, such as a bandwidth request, or an ACK message, have no payload, such as a Feedback header referred to later.
In general, the payload of the DL or UL MAC PDU can include any of a set of predefined MAC messages (Mmsg). These include a Channel measurement Report Request (REP-REQ) and a Channel measurement Report Response (REP-RSP) which are referred to later. Furthermore, a subheader may be included immediately after the generic MAC header of a MAC PDU. One purpose of this subheader is to define a fast feedback channel, which is also referred to later.
The same basic structure is also employed in OFDMA, although there are some differences in the PHY layer signalling. The OFDMA physical layer divides the available OFDM symbols and component sub-carriers (see FIG. 3) into distinct logical and physical subchannels, allowing multiple bursts to co-exist or overlap in each time interval. On the downlink, a single burst may be shared by several users (subscriber stations) but on the uplink, each burst generally corresponds to a single user.
In FIG. 3, the frame can be considered to occupy a given length of time and a given frequency band, the time dimension being denoted in FIG. 3 by “OFDMA symbol number”, and the frequency dimension by “subchannel logical number” (each subchannel is a set of the sub-carriers referred to above). Each frame is divided into DL and UL subframes, each being a discrete transmission interval. They are separated by a Transmit/Receive and Receive/Transmit Transition Guard interval (TTG and RTG respectively). Each DL subframe starts with a preamble followed by the Frame Control Header (FCH), the DL-MAP, and, if present, the UL-MAP. The FCH contains the DL Frame Prefix (DLFP—see also FIG. 2) to specify the burst profile and the length of the DL-MAP. The DLFP is a data structure transmitted at the beginning of each frame and contains information regarding the current frame; it is mapped to the FCH. Simultaneous DL allocations can be broadcast, multicast and unicast and they can also include an allocation for another BS rather than a serving BS. Simultaneous ULs can be data allocations and ranging or bandwidth requests.
The DL-subframe includes a broadcast control field with a DL-MAP and UL-MAP, by which the BS informs the receiving device of the frame structure. The MAP is a map of bandwidth allocation in the frame and also contains other PHY signalling related messages. It consists of Information Elements (IE) each containing a connection ID. The map IEs inform mobile stations to which burst(s) they have been assigned to receive information. Thus, in a TDD and FDD mode network, bandwidth allocation means the allocation of resources (slots) within frames. The DL-MAP and UL-MAP are examples of management messages broadcast by the BS (that is, transmitted to all subscribers). The areas marked UL burst#1 to #5 in FIG. 3 represent allocations to respective users for their data transmissions to the BS. In this example, each burst extends over the whole of the time dimension (the duration of the UL subframe) but occupies a different part of the available frequency dimension (subcarriers).
The UL-subframe further includes so-called channel quality indication channels (CQICH) which are used by the MSs to feed back channel quality to the BS as mentioned below.
A connection between a base station and subscriber station (more precisely, between MAC layers in those devices—so-called peer entities) is assigned a connection ID (CID) and the base station keeps track of CIDs for managing its active connections or service flows. A service flow could represent, for example, a voice call conducted by the user of the MS.
In wireless communication systems, and in particular OFDM and OFDMA systems employing adaptive channel encoding and modulation, it is very important for each MS to inform its BS of the quality of its connection (also called channel quality) and of other parameters. One way of achieving this, known in the art, is for each MS to send sporadic messages to the BS. The way generally used to do this is by sending a MAC management message (Mmsg) to report to the BS, such as the REP-RSP mentioned above. Such a Mmsg may be solicited, that is requested by the BS by sending a REP-REQ, or may be unsolicited. An alternative method of sending such a message is to use a special MAC header without a PDU payload, for example, a feedback header sent by an MS either as a response to a Feedback Polling IE or as an unsolicited feedback.
A second way for the MS to inform the BS of the connection quality is to use a fast feedback (FFB) dedicated channel such as the CQICH mentioned earlier, on which each MS periodically reports to the BS. Here, “dedicated” means that a slot (or part of a slot) is allocated to just one connection without being shared by other users in any one frame; however, the same physical resource may be used for a different connection (user) in a subsequent frame. Allocation of bandwidth to a FFB channel takes place either through the FFB MAC subheader mentioned above, or through a CQICH Control IE and CQICH Allocation IE. These allocations can be made on a one-off basis (FFB MAC subheader), or in other words, lasting for only the current frame, or persistent basis (CQICH Allocation IE) and the persistent allocations may occur periodically, i.e. every frame or every n frames. Various ways are available to make this allocation as is known to those skilled in the art.
It is also possible to combine these two approaches so that each MS may send sporadic Mmsgs in addition to using the dedicated feedback channel for reporting during the same session.
However, there are problems with the above approaches as follows. Using Mmsg on a regular basis incurs a high processing overhead, especially when the BS is serving many mobile stations. When using a FFB channel, whilst the overhead is potentially lower, the BS has to allocate a dedicated resource to each MS even if the MS has nothing of significance to report (i.e. no change in conditions). In some applications, a number of MSs may need to report to the BS non-periodically but frequently, in which case neither the Mmsg nor the dedicated channel approach will provide an optimum mechanism. In such a case, the BS will not know when and how many of the mobile stations will report at any one time, and only knows which MSs may report. Thus, the BS has to choose between allocating dedicated channels requiring bandwidth resources (slots) for all mobile stations, thus wasting bandwidth, or incurring additional overhead by making frequent use of Mmsgs.